doc: document changes to cmd/vet for 1.18

[findleyr]

Filing this as a separate issue from #47694 as I'm not sure if cmd/vet changes would get caught by our automated processes, and this should be a release blocker. For some reason I only just connected the dots that this needs to get done.

We made many changes to vet analyzers for 1.18 to support generics. Not all of them are significant enough to warrant documentation, but there are certainly some non-trivial changes that should be in the release notes. For example, #49038 had an associated proposal.

I can try to get this done in the next day or two. @timothy-king I'll probably send the reviews your way.

CC @golang/release @aclements

[gopherbot]

Change https://golang.org/cl/369979 mentions this issue: doc: document cmd/vet changes for generics in 1.18